# [Proj] Understanding deformation model usage

stevejking stevejking at talktalk.net
Tue Feb 27 08:47:59 EST 2018

```Yes better understood now。Thanks Kristian

Kristian Evers-2 wrote
> Steve,
>
> I was considering just calling them t_a and t_b or something like that to
> illustrate that what really matters is the difference between the two
> since it is all relative. Which t comes first dictates how the grid is
> The way I think about it is that we are moving a coordinate at time t_obs
> backwards or forwards in time to t_epoch. In the example in the docs the
> last step is applying the deformation model on a coordinate at
> t_obs=2000.0 which we want to move back in time to t_epoch=1994.704 (which
> is the epoch of the Danish ETRS89 realisation). After the deformation
> model has been applied we have subtracted ~5 years' worth of land uplift.
>
> I hope that helps you understand this better.
>
> /Kristian
>
> -----Oprindelig meddelelse-----
> Fra:

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>  [mailto:

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> ] På vegne af stevejking
> Sendt: 27. februar 2018 09:34
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> proj at .maptools

> Emne: Re: [Proj] Understanding deformation model usage
>
> I can see t_obs is always the *grid_epoch, except when you do an inverse
> (by
> swapping t_epoch and t_obs).
>
>
>
> stevejking wrote
>> Kristian,
>> I think I follow your (in development) user guide example here. Very
>> useful
>> thanks.
>> https://kbevers.github.io/usage/operations/transformations/deformation.html
>>
>> But if the velocity shifts are tied to Cartesian coordinates, surely this
>> is
>> only the average velocities at the coordinate at a moment in time. There
>> is
>> no parameter to specify a *grids_epoch.
>>
>> Eg. What to do If my grid is Cartesian coordinates and velocities at
>> epoch
>> 2010.0, t_obs=2000.0 and t_epoch=2017.0?
>> Can be achieved by ensuring in individual steps that t_obs or
>> t_epoch always equals *grids_epoch ? I think that’s what your example
>> does?
>>
>> *grids_epoch is a made up option to illustrate.
>>
>> Steve
>>
>>
>>
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>
>
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